Lost foam casting of metals and alloys involves pouring molten metal or alloy onto a vaporizable polystyrene pattern residing in a bed of foundry sand so that the molten metal vaporizes the pattern and displaces it in the foundry sand to form a casting. Lost foam casting is described in such patents as U.S. Pat. Nos. 4,085,790; 4,616,689; and 4,874,029. In the past, the polystyrene pattern has been coated with an thermally insulative, gas permeable refractory layer or coating prior to being embedded in the foundry sand. The gas permeable refractory coating allows pattern decomposition vapors to vent into the foundry sand and also helps prevent collapse of the sand as the pattern is vaporized. U.S. Pat. Nos. 4,482,000 and 4,448,235 describe polymer-modified refractory coatings for use on polystyrene lost foam patterns to reduce entrapment of pattern decomposition vapors in the molten metal as it replaces the pattern.
In the lost foam casting of aluminum alloys using polystyrene patterns having core-forming passages that are filled with foundry sand when the pattern is embedded in the foundry sand bed, the gaseous decomposition products of the foam pattern have been found to cause gas holes and so-called fold defects in the cast component proximate the core-forming passages, especially those passages located remote from the initial melt/pattern contact region. The gas holes are usually rounded and detectable upon x-ray examination of the cast component and affect casting quality in dependence on their size and quantity. Fold defects generally are not detectable on x-ray examination but are found when castings are test fractured. Fold defects generally are considered nearly two dimensional, constituting very thin void regions or seams of unbonded metal forming a plane of weakness that results in low strength and low ductility of the casting. In the past, attempts to reduce fold defects in lost foam aluminum alloy castings have involved casting the molten aluminum alloy at very high temperatures of up to 1450 degrees F, for example. However, although fold defects may be reduced by use of high melt casting temperatures, they nevertheless still have been found to be present in a problematic percentage (e.g. 10-50%) of lost foam castings that are produced in a high volume lost foam production environment such as, for example, cast automotive cylinder blocks and heads. Moreover, use of such high melt casting temperatures produces coarse grained lost foam castings that reduces their mechanical properties.
Production engineers have been constantly attempting to eliminate or substantially reduce fold defects in lost foam castings but, to-date, there has been no generally reliable solution to the problem of fold defects. As a result, lost foam cast components of less than optimum quality and higher than desired lost foam production costs still are encountered.
An object of the present invention is to provide method and apparatus for lost foam casting of aluminum and its alloys as well as other metals and alloys using a vaporizable pattern in a manner to substantially reduce or eliminate fold defects in the cast component.